It's the amount of influence you have in the network. The more SP, the bigger your vote. You can earn it by posting, and you can buy it via blocktrades, poloniex.com or bittrex.com

Here's a link to the FAQ:

https://steemit.com/faq.html#What_is_the_difference_between_Steem__Steem_Power__and_Steem_Dollars